Tamarix sp., are woody members of the the Tamarisk Family (Tamaricaceae). Worldwide, there are 54 species of Tamarix. There are several hypotheses about the origin of the common name Tamarisk. Some people suggest that the name is derived from the Tambre (Tamariz) River in Spain, others from the Tamaro River in Nepal , and others from the Hebrew word tamaruk . The common name salt cedar refers to the cedar-like foliage of the plant, and their ability to grow in saline or alkaline soils. Tamarix species are important invasive species in parts of the southwestern United States.
Tamarix species are spreading shrubs or small trees generally growing about five to twenty feet (1.5 to 6.0 meters) high. A single plant produces numerous slender branches that exhibit small, alternate scale-like leaves. Tamarix species produce a deep root system that can tap into ground water up to 30 meters beneath the surface and can extend long distance laterally to find water source up to 50 meters away. Tamarix may grow rapidly with up to three to four meters of branch growth recorded in a single growing season by some species.
Tamarix generally flowers in its third year or later, but may flower during the first year. Buds generally break dormancy in February or March. The flowers are most abundant between April and August, but may be found any time of the year in desert areas. Tamarix can flower continuously under favorable environmental conditions, but the flowers require insect pollination to set seed. The pale pink to white flowers are small, perfect and regular, arranged in spike-like racemes. The distinct petals and sepals occur in fours or fives. The fruit is a capsule. Mature Tamarix plants are capable of producing more than 600,000 tiny, wind-dispersed seeds per year. The seeds are short-lived, surviving less than two months in summer. Seeds have no dormancy requirements so they may germinate in less than 24 hours. Seeds can germinate while they are floating in the water. Seedlings establish most frequently in soils that are seasonally saturated at the surface. Seed predation is relatively low because of (1) rapid time to germination, (2) short dormancy period, (3) their small size, and (4) their low protein content. Seedling densities can exceed 16,000 per square meter. Early growth of seedlings is slow, but older seedlings may grow rapidly- up to a foot a month in early spring. Seedlings are tolerant of a variety of environmental conditions including submergence, saline soils, and drought.
Tamarix resprouts from roots if the top portion of the plant is damaged or removed. Resprouting is an adaptation to disturbances such as flooding and fires. This characteristic has important implications for control efforts.
The genus Tamarix is native to a zone stretching from southern Europe and north Africa through the Middle East and south Asia to China and Japan, and there are a few species in disjunct parts of Africa. Tamarix is now established in many moist spots in the desert regions of the western U.S. It appears to grow best in saline soils but it is tolerant of a wide variety of environmental conditions
Introduction into the United States
Eight species of Tamarix, including T. aphylla, T. chinensis, T. gallica, T. parviflora, and T. ramosissima, were introduced into the United States from Asia in the 1800s. Originally the Spaniards were blamed for the introduction of alien species of Tamarix, but scientists now believe that nurserymen sold the plants to Old American Nursery in Philadelphia. At least three species of Tamarix were sold in 1854 to provide a variety of functions including setting up wind breaks, creating shade, stabilizing eroding streambeds, and as ornamental shrubs. Tamarix was first reported outside of cultivation in the 1870s and the greatest invasion occurred between 1935 and 1955.
Effects of Tamarix invasion
Tamarix has five main impacts on the local environment once it becomes established (1) increased soil salinity, (2) increased water consumption, (3) increased wildfire frequency, (4) increased frequency and intensity of flooding and (5) altering plant community and wildlife habitat.
Tamarix readily takes up solutes from the soil and then deposits them above the ground either from its salt glands or by dropping its leaves which increases the concentration of salts in the soil. There may be an allelopathic effect if surrounding plants are unable to grow in these high salt concentrations. For example, salt cedar has been shown to tolerate up to 36,000 parts per million (ppm) salt salinity, whereas native floodplain species such as willow and cottonwood can only tolerate up to 1500 ppm.
Increased water consumption
Water use by Tamarix is generally considered to be high. A single large plant can absorb 200 gallons of water a day, but evapotranspiration rates often vary throughout the day and depend on conditions. In general, the longer a community has been invaded by salt cedar, the lower the water table is beneath the surface.
Increase wildfire frequency
Wildfires become more frequent within stands containing high densities of Tamarix. The deciduous nature and high population densities of Tamarix both contribute to a heavy fuel load in infested areas. Between 1981 and 1992, fires burned 35% of Tamarix infested areas on the lower Colorado River floodplain. However, during this same time frame, fires burned two percent of the native plant species in a nearby area. Tamarix promotes a fire rotation of about 10 to 20 years. The fire tolerance of Tamarix (e.g., ability to resprout) coupled with the fire intolerance of many native shrubs in the southwestern deserts allows Tamarix to dominate native plant communities in a relatively short time period.
Increased frequency and Intensity of flooding
The density of Tamarix stands has been positively correlated with the frequency of flooding. As the density of Tamarix stands increase the channel that water flows through decreases which increases the rate of water flow and the chances for flooding. A Tamarix infestation along the Brazos River in north central Texas began in 1941. By 1979 the river's depth had changed from 18.4 to 10.2 feet and the width had changed from 515 to 220 feet.
Altering plant community and wildlife habitat
Tamarix invasion has serious consequences on the structure and stability of native plant communities. The decline of riparian stands of cottonwood along the Rio Grande River in New Mexico is partially attributable to the invasion of Tamarix. Dense stands of exotic plants along the floodplain can limit the number of germination sites that are suitable to cottonwood. In Australia, Tamarix displaces native plant species, resulting in a relatively few species of introduced and salt-tolerant plants dominating the of native vegetation.
The suitability of Tamarix as wildlife habitat has been a subject of considerable debate. It is used by wildlife in its native range in the old world. For example, elephants in Namibia, Africa prefer to forage on Tamarix usneoides. However, outside of its natural range in the southwestern United States it generally provides unsuitable habitat for most wildlife because neither its foliage, flowers, or seeds have any significant food value unlike native species such as mesquite. One exception is the the exotic honeybee, Apis mellifera, which utilizes the pollen of Tamarix. Tamarix provides cover for some species, particularly birds such as doves and Mississippi Kites. Some passerine birds nest in Tamarix stands.
The value of Tamarix to wildlife appears to vary geographically. For example, the use of Tamarix by birds was high on the middle Pecos River, intermediate on the lower Rio Grande River, and very low on the lower Colorado River. Birds may be enticed to use Tamarix along the Pecos River if they are attracted by seed producing shrubs and annuals either within or adjacent to the Tamarix stand.
Economic effects of Tamarix invasion
Invasion by Tamarix can have negative effects on irrigation, municipal water, flood control, hydropower, wildlife habitat, and river recreation. Benefits of increased Tamarix cover includes dove hunting and reduced rates of sedimentation. A vatiety of Tamarix control methods are currently being used.
Estimates of water loss (the difference between Tamarix use and native vegetation use) on the Colorado River were as high as three acre-feet per year with up to 568,000 ac-ft/yr lost lost along the Colorado River each year. The estimated costs of water lost by Tamarix from the Bonneville Unit of the Central Utah Water Project is $27 million annually. Based on estimates of 1.16 million acres (0.47 million hectares) to 1.6 million acres (0.65 million hectares) infested, the invasion of Tamarix in the western United States will cost from $7 billion to $16 billion in lost ecosystem functions over the next 55 years. This loss amounts to an average of $6,300 to $10,000/ac ($15,600 to $24,600 per hectare) of land infested with Tamarix.
References and Further Reading
- Technical Information About Tamarix (Salt Cedar). Non-native Freshwater Plants.
- Salt Cedar Colorado Weed Management Association.
- Tamarix sp. U.S. Forest Service
- A Brief Overview of the Impact of Tamarisk Infestation on Native Plants and Animals National Biological Service.
- Tamarisk, saltcedar, athel tree (Tamarix spp.) Arizona-Sornora Desert Museum.